1. Field of the Invention
This invention relates to a back-illuminated solid-state imaging device and a method for manufacturing the same.
2. Background Art
Front-illuminated solid-state imaging devices have been conventionally developed. In a front-illuminated solid-state imaging device, a multilayer interconnect layer is provided on the frontside of a semiconductor substrate, and color filters and microlenses are provided on the multilayer interconnect layer. Furthermore, photodiodes are formed in the surface portion of the semiconductor substrate, and transfer gates are formed in the multilayer interconnect layer. For instance, the photodiode is formed from an n-type diffusion region, and partitioned for each pixel by p-type isolation layers. Light incident on the semiconductor substrate through the microlens, the color filter, and the multilayer interconnect layer from above is photoelectrically converted by the photodiode to generate electrons, which are read through the transfer gate.
In such a front-illuminated solid-state imaging device, externally applied light is incident on the semiconductor substrate through the multilayer interconnect layer, hence resulting in low light use efficiency. Thus, reduction in pixel size decreases the amount of light incident on the photodiode of each pixel, causing the problem of decreased sensitivity. Furthermore, reduction in pixel size also decreases the distance between pixels. Hence, light incident on a pixel may be diffused by the metal interconnect in the multilayer interconnect layer and incident on another pixel, causing the problem of color mixture. Color mixture decreases color resolution and prevents distinction of subtle color difference.
To solve these problems, a back-illuminated solid-state imaging device is proposed, which allows light to be incident on the backside of the semiconductor substrate, or on the side where no multilayer interconnect layer is provided (see, e.g., JP-A-2003-031785 (Kokai)). In a back-illuminated solid-state imaging device, externally applied light is incident on the semiconductor substrate without the intermediary of the multilayer interconnect layer, hence achieving high light use efficiency and high sensitivity.
However, in the back-illuminated solid-state imaging device, because light is directly incident on the semiconductor substrate without passing through the multilayer interconnect layer, obliquely incident light also reaches the semiconductor substrate without being blocked by the metal interconnect. This increases the amount of light incident on the isolation layer for isolating between photodiodes. Incidence of light on the isolation layer causes photoelectric conversion in the isolation layer and generates electrons, which flow into the photodiode adjacent to the isolation layer. Consequently, for instance, light incident on a red pixel may be incident on the isolation layer located between the red pixel and a blue pixel, and electrons generated by photoelectric conversion in this isolation layer may flow into the photodiode of the blue pixel and be detected as blue light. This causes color mixture. Reduction in pixel size increases the area ratio of the isolation layer to the photodiode, hence aggravating color mixture.